1. Field of the Invention
This invention relates to a solid-state imaging apparatus, and more particularly to a solid-state imaging apparatus with a solid-state imaging device having an improved signal processing portion which performs an electronic shutter operation.
2. Description of the Related Art
Solid-state imaging devices using CCDs (charge-coupled devices) have many features including compactness, lightweight, high reliability, and easy maintenance and are applied to a wide variety of electronic cameras. Recently, solid-state imaging devices for high-definition television (HD-TV) camera have been developed and put to practical use.
With the HD-TV camera, high-definition images are produced on a multi-pixel wide screen (an aspect ratio of 9:16), so that when a moving subject is picked up with the HD-TV camera, the motion resolution deteriorates seriously owing to a time (photoelectric conversion time) during which the photosignal detected by the imaging apparatus is converted to an electric signal and stored in a pixel, degrading the quality of the picture significantly. To overcome this shortcoming, an electronic shutter operation has been applied which varies the photoelectric conversion time of photosensitive pixels on the solid-state imaging device. Nevertheless, in an electronic shutter operation, since the photoelectric conversion time is short, the amount of signals decreases, causing the problem of degrading the signal-to-noise ratio. Hereinafter, this problem will be explained briefly.
In a standard operation of the HDTV system, the photoelectric conversion time of the photosensitive pixels of the imaging device is determined to be 1/60 sec. When the subject moves over the area of a single pixel during this time, the moving subject results in a blurred image, degrading the motion resolution. To overcome this drawback, an electronic shutter operation is effected with a photoelectric conversion time of, for example, 1/600 sec (normally, the time may vary from 1/125 to 1/1000 sec). In this case, since the photoelectric conversion time is 1/10 sec and therefore the amount of signals is as small as 1/10, the signal-to-noise ratio is seriously degraded.
As mentioned above, in the standard operation of present-day television cameras, when a moving subject is picked up, the deterioration of the motion resolution takes place, degrading the quality of the picture significantly. Particularly, since a next-generation HD-TV camera produces high-definition images on a wide screen, this further degrades the picture quality due to the deterioration of the motion resolution. To overcome this problem, an electronic shutter operation as an option mode can be considered. This approach, however, introduces problems including an increase in noise as a result of an increased gain due to a decrease in the sensitivity and a decreased focussing depth as a result of opening the diaphragm.